Method for optically detecting chemical species contained in condensed media

ABSTRACT

The invention relates to a device for detecting chemical species present in a condensed medium, comprising means for determining the wavelength and the intensity values which are characteristic of electromagnetic transmission signals which are backscattered in response to a plurality of electromagnetic excitations, which have distinct wavelengths, of at least one chemical species which can be contained in the condensed medium; laser means producing a beam in order to excite the condensed medium; means for recording the wavelengths and the intensity values of backscattered electromagnetic transmission signals; and comparing and determining means for comparing the recorded intensity value of the electromagnetic signal, which is backscattered by the medium at a determined characteristic intensity value, with at least one corresponding transmission wavelength and excitation wave length.

[0001] The present invention pertains to a method and a device for thedetection of chemical species present in condensed medium.

[0002] The envisaged fields of application are notably those ofmonitoring the composition of the aqueous effluents discharged by awater purification station or any other industrial operation dischargingeffluents.

[0003] Another envisaged field of application is that of monitoring theformation of a chemical compound in an industrial production process.

[0004] The monitoring of industrial discharges in nature in liquid formis generally performed visually and by analysis of discharged liquidsamples according to a specific method for each chemical species beinginvestigated. Furthermore, when monitoring a large expanse of aqueouseffluent that might comprise chemical species not uniformly distributedon said expanse, it is necessary to collect multiple samples atdifferent sites in order to localize the origin of the production ofsaid species. The time required for the analysis of the sample and thereplacement rate of said effluent affect the diagnosis with regard tothis localization.

[0005] Moreover, the detection of the appearance of a reaction compoundby the collection of samples of the reaction medium present a doubledrawback. First of all, the reaction is affected by the collection ofsaid sample and, second of all, the longer the time required for theanalysis of said chemical compound in relation to the reaction rate andthe lower the degree to which the monitoring of the reaction ispossible.

[0006] In order to resolve this drawback, it has been envisaged todetect the presence of chemical species by spectroscopic meanscomprising means of electromagnetic excitation oriented to the medium tobe analyzed and means for the spectroscopic analysis of the signalback-scattered by the surface of said medium. In this manner, thechemical species are identifiable essentially instantaneously withoutdisturbing the medium.

[0007] However, the means of electromagnetic excitation directed on themedium to be analyzed excite a surface of the medium. Thus, on the onehand, the incident signal must be sufficiently strong so as to exciteall of the species of the surface and, on the other hand, the means ofdetection must be extremely sensitive in order to detect the spectra ofsaid chemical species of said surface.

[0008] A problem which exists and which the present invention intends toresolve is that of providing a method for the detection of chemicalspecies present in a condensed medium which not only makes it possibleto precisely detect the nature of the chemical species present in saidcondensed medium with less costly detection means, but also which enableexcitation of the surface of said condensed medium with means of reducedpower and thus equally less costly.

[0009] For this purpose, the present invention proposes a method for thedetection of chemical species comprising the following steps: onedetermines the characteristic wavelengths and intensity values ofback-scattered electromagnetic signals in response to a multiplicity ofelectromagnetic excitations of distinct wavelengths, of at least onechemical species which can be contained in said condensed medium; oneexcites successively a multiplicity of surface elements of a portion ofthe surface of said condensed medium with a laser beams the tunablewavelength of which can take on at least the values of said distinctwavelengths of said multiplicity of electromagnetic excitation; onerecords successively the wavelengths and the intensity values of theelectromagnetic emission signals back-scattered by each of said surfaceelements in response to the electromagnetic excitations produced by saidbeam; one compares at at least one excitation wavelength and at at leastone corresponding emission wavelength the recorded intensity value ofsaid back-scattered electromagnetic signal of each of said surfaceelements at said characteristic intensity value determined from saidback-scattered electromagnetic signal of said chemical species whichcould be contained in said surface portion; and one determines thepresence of said chemical species in each of said surface elements whensaid recorded intensity value of said electromagnetic signalback-scattered by said surface element is greater than a thresholddefined at least by said given characteristic intensity value of saidback-scattered electromagnetic signal of said chemical species.

[0010] Thus, the method is based on the analysis of back-scatteredelectromagnetic signals stemming from the fluorescence of chemicalspecies excited by a beam from laser means, said signals beingcharacteristic of said chemical species. At given excitation wavelengthsof the laser beam, the targeted chemical species diffuse theelectromagnetic signals the intensities and wavelengths of which arecharacteristic of said species. In this manner, by exciting a givenchemical species with laser means and by varying the excitationfrequency one obtains in response back-scattered signals the wavelengthsand intensities of which are characteristic.

[0011] When one determines the characteristic wavelengths andintensities of back scattered signals associated with one or moreincident wavelengths determined for a given chemical species, the methodaccording to the invention enables detection of the presence of saidgiven chemical species on a portion of the surface of the more or lessextended condensed medium by decomposing said surface portion intosurface elements and by exciting said surface element with a laser beamat said given incident wavelengths and back-scattered signal intensitiesrecorded at the characteristic wavelengths and intensities of thesignals of said species. In this manner, the laser beam can be applieddirectly on the surface of the condensed medium and its intersectionwith said surface determines said surface element. When the wavelengthsand the intensity values coincide or if the wavelengths coincide and theintensities are greater than a given threshold, said species isconsidered to be contained in the condensed medium that has beenexcited. Quite obviously, the threshold value is adjusted as a functionof the noise level of the detector system. The surface elements of anentire surface portion are therefore capable of being analyzedindependently from each other with a high level of precision because thelaser beam is concentrated on a surface element of a surface portion andone records the wavelengths and the intensity values of the signalsback-scattered by said surface element. In this manner, the power of thelaser means can be reduced and the detection means can be less sensitivewhile still preserving a high level of detection.

[0012] As a result of the directivity of the laser means, the compoundsof the surface elements are excited successively and for each surfaceelement the wavelength of the incident radiation is made to vary and theback-scattered signals are collected so as to establish the presence orlack thereof of the given chemical species in all of the surfaceelements of said surface portion.

[0013] However, as will be explained in greater detail below in thecontinuation of the description, a chemical species can present multiplecharacteristic emission signals at different wavelengths in response toa single excitation wavelength. In this case, the incident radiationwould be tuned solely on this excitation wavelength if only thischemical species is being investigated.

[0014] One records successively the direction of said beams of the lasermeans advantageously for each surface element of said surface portion soas to establish the reference point coordinates of the origin of saidback-scattered electromagnetic emission signals, by means of which oneobtains the position of said chemical species in said surface portion.In fact, since the distance that separates the laser means from thesurface portion is known, the relative positions of each surface elementare determined by the relative angular offsets of the direction of thebeams if the laser means pivot around a fixed point. Thus, there isassigned to each given position corresponding to a surface element saidback-scattered electromagnetic emission signals corresponding to thissurface element in a manner such as to establish the coordinates of theposition of said chemical species.

[0015] According to a particularly advantageous implementation of theinvention, one moreover determines the concentration of said chemicalspecies present in said medium by measuring the quantity of energyemitted by said back-scattered electromagnetic emission signals. In thismanner, because for a given wavelength the energy of the back-scatteredsignal is a function of the number of photons emitted and thus afunction of the quantity of chemical species that diffuses the incidentradiation, it is possible after calibration to correlate the energy ofthe back-scattered signal and the quantity of said chemical species.

[0016] According to a preferred mode of implementation of the invention,one records in parallel the intensity values of said back-scatteredelectromagnetic emission signals and their corresponding wavelength isrecorded as well. In this manner, it is possible to very rapidly recordthe spectra of the chemical species present in the surface portion.

[0017] Thus, for each surface element containing the chemical speciesthat emits a back scattered signal, the position and the intensity ofsaid signal are detected by detection means simultaneously with themeasurement of the wavelength of said signal. Thus, one obtains in afive-dimension base, the position of the surface element with twodimensions, the excitation wavelength, the wavelength of theback-scattered signal and the intensity of said signal. The presence ofthe chemical species and its position in the surface portion are therebydetermined.

[0018] A second object of the present invention is to propose a devicefor the detection of chemical species present in a condensed medium,which device comprises: means for determining the characteristicwavelengths and intensity values of back-scattered electromagneticemission signals in response to a multiplicity of electromagneticexcitations of distinct wavelengths of at least one chemical speciesthat could be contained in said condensed medium; laser means producinga beam for successively exciting a multiplicity of surface elements of asurface portion of said condensed medium according to wavelengthscapable of taking at least the values of said distinct wavelengths ofsaid multiplicity of electromagnetic excitations; means for successivelyrecording the wavelengths and the intensity values of electromagneticemission signals back-scattered by each of said surface elements inresponse to the electromagnetic excitations produced by said beam; andcomparison and determination means for comparing at at least oneexcitation wavelength and at least one corresponding emission wavelengththe recorded intensity value of said electromagnetic signalback-scattered by each of said surface elements at said givencharacteristic intensity value of said back-scattered electromagneticsignal of said chemical species that could be contained in saidcondensed medium and for determining the presence of said chemicalspecies in each of said surface elements when said recorded intensityvalue of said electromagnetic signal back-scattered by said surfaceelements is greater than a threshold defined at least by said givencharacteristic intensity value of said back-scattered electromagneticsignal of said chemical species.

[0019] Thus, a characteristic of the device is based on the combinationof the means producing a coherent electromagnetic beam oriented to asurface element of said surface portion at given wavelength values andmeans for recording the intensity and wavelength values of theback-scattered signals, these means being combined in turn withcomparison and determination means, which compare said recorded valueswith given wavelength and intensity values of the chemical species thatcould be contained in the condensed medium in order to determine thepresence or lack thereof of said species.

[0020] According to a particular mode of implementation of theinvention, said laser means comprise: a pump laser associated with afrequency doubler; and a parametric oscillator to which said pump laseris coupled in a manner such as to emit radiation the tunable wavelengthof which is in the range between 200 and 800 nm. In this manner, a largenumber of chemical species can be identified and distinguished from eachother.

[0021] According to a particular advantageous characteristic, said lasermeans producing a beam comprise orientation means of said beam forexciting said multiplicity of surface elements of said surface portionof said condensed medium in a manner to analyze the back scatteredelectromagnetic emission signals stemming from each of said surfaceelements and of determining the presence of at least one of saidchemical species in each of said surface elements of said surfaceportion.

[0022] As will be explained in greater detail in the continuation of thedescription, the displacement means comprise mobile mirrors fororienting the beam of each of the surface elements, with thesedisplacement means being controlled by control means.

[0023] As a result of the determined position of said mirrors, it ispossible to determine the direction of the beam and in a preferentialmanner the device according to the invention comprises means forsuccessively recording the direction of said beam of the laser means foreach surface element of said surface portion so as to reference thecoordinates of the origin of said back-scattered electromagneticemission signals by means of which one obtains the position of saidchemical species in said surface portion. In this manner, the surfaceelement by surface element sequential archiving enables creation of amatrix comprising at each point the spectrum of said chemical species.

[0024] In a particularly advantageous manner, the detection devicecomprises recording means comprising a spectrometer coupled to aphotodetector matrix in a manner such as to record in parallel theintensity values of said back-scattered electromagnetic emission signalsand to record their corresponding wavelength.

[0025] Other specific characteristics and advantages will becomeapparent from the description below of specific modes of implementationof the invention presented in a nonlimitative indicative manner withreference to the attached drawings in which:

[0026]FIG. 1 is a schematic view showing the detection device accordingto the invention;

[0027]FIG. 2 is a representation of a spectrum which could be obtainedby means of the device according to the invention.

[0028]FIG. 1 illustrates the detection device according to the inventionwhich presents laser means 10 forming an excitation beam 12, means 14for recording a back-scattered signal 16 and comparison anddetermination means 18 contained in the central unit of a computer 20.Moreover, the central unit comprises programs for controlling theentirety of the device according to the invention.

[0029] The laser means 10 comprise a pulsed laser 22 of the NdYAG typecoupled to a frequency converter unit 24, for example a frequencydoubler or tripler, such that the first beam 26 stemming from it isoriented to an optical parametric oscillator 28 providing at least onesecond beam 30 which is directed to a second frequency doubler 32. Theparametric oscillator 28 makes it possible to vary in a continuousmanner the wavelength of the second beam 30.

[0030] In a particularly advantageous manner, said laser means comprisea pumping source operating in femtosecond mode and forming a compactsystem. These laser means have the advantage of being available at lowcost.

[0031] The tunable laser means 10 enable provision of an excitation beam12 with a section of several cm² at a distance of 100 m and thewavelength of which can vary at least between 220 and 750 nm, wavelengthinterval within which the chemical species that could be excited presentcharacteristic spectra.

[0032] The pump laser 22 can be advantageously replaced by a diodesystem that presents the same advantages.

[0033] The excitation beam 12, originating from said laser means 10,traverses semitransparent means 34, for example, a semitransparent prismor strip, and then encounters means 36 for the displacement of theexcitation beam 12 constituted by two orientable mirrors which reflectthe beam onto a condensed medium 38 that could contain the chemicalcompounds. In this manner, the intersection of the excitation beam andthe surface of the condensed medium form at 100 m of distance a surfaceelement of several cm², for example, 3 cm².

[0034] Said chemical compounds can emit a back-scattered electromagneticsignal 16 in response to the excitation induced by the excitation beam21, said back-scattered electromagnetic signal 16 prints the sameoptical path as the excitation beam 12 up to the transparent means 34that orient the back-scattered signal 16 to the recording means 14.

[0035] These means 14 comprise a spectrometer 40 capable of determiningthe wavelengths of the back-scattered electromagnetic signals 16 andcoupled to detector means 42 constituted by a matrix of photoelectricsensors, for example, CCD, capable of determining the intensities at aposition of the back-scattered signals 16. Moreover, the recording means14 are linked to the center unit of the computer 20 which has a memorythat can store simultaneously, notably, the wavelength of theback-scattered signal 16 and its intensity.

[0036] The central unit of the computer 20 is also linked to the lasermeans and to the means 36 for displacement of the excitation beam so asto be able to control them by means of command programs. Furthermore,the direction of the excitation beam 12 which determines the position ofan excited surface element is also stored in the memory of the computer20 along with the intensities and wavelengths of the back-scatteredwaves. According to a particular mode of implementation of theinvention, the position of the surface elements can be determined byreferencing the coordinates of the pixels of the photoelectric sensorpixels which is located in the focal plane of the optical system.

[0037] In this manner, the computer 20 can control for a given positionof the means 36 for displacement of the excitation beam 12, the lasermeans 10 in a manner such as to vary as a function of time thewavelength of the excitation beam 12, for example, between 250 and 450nm. The computer 20 simultaneously stores in its memory the determinedposition of the back scattered signal 16, determined by the means 36 fordisplacement of the beam, the wavelength and back-scattered signalintensity 16 for each of the wavelength values of the excitation beam12. The command programs then command the movement of the displacementmeans 36 in a manner such that the excitation beam 12 targets thesurface element of the surface portion 38 contiguous to the preceding soas to perform the same spectral scanning. This operation is repeated soas to cover the entire surface portion 38.

[0038] Thus, five variables are stored in the memory space of thecomputer 20: three variables characterize the chemical species presentin the surface element that the excitation beam 12 excites and twovariables characterize the position of said surface element in relationto the surface elements whose coordinates are referenced by the relativepositions of the displacement means 36 and stored in the memory of thecomputer 20.

[0039] The characterization of the chemical species will be describedwith reference to the means of FIG. 2 illustrating the spectra of amixture containing at least two aromatic hydrocarbons: anthracene and abenzopyrene.

[0040] The spectrum of the chemical species is characterized by thevariable wavelength of the back-scattered signal 16 plotted on theabscissa axis 50 and by the variable intensity of the back-scatteredsignal plotted on the ordinate axis 52. Curve 54 represents theintensity of the back-scattered signals and their wavelength in responseto an excitation with a wavelength of 380 nm. Curve 54 exhibits threepeaks 56, 58, 60 at 411, 437 and 457 nm, respectively, characteristic ofbenzopyrene. Curve 62, in response to an excitation at 390 nm, alsoexhibits three peaks, 64, 66 and 68 at 450, 425 and 396 nm,respectively, characteristic of anthracene. Furthermore, the excitationat 400 nm generates an essentially flat curve 70 which does not allowany characterization.

[0041] Looking at these curves 54, 62, 70, it can be understood that thedetection of a given chemical species, for example, anthracene, in agiven condensed medium, can be performed by comparing the intensity ofthe signal back-scattered at 411, 432 and 457 u for an excitationproduced at 390 nm and determining in it the presence of anthracene if,for example, the intensity of the signals at all the wavelengths isproportional to the intensity of the characteristic signals ofanthracene.

[0042] In contrast, it can be understood that an excitation at 400 nm ofthe condensed medium does not allow distinction of the presence ofanthracene nor that of benzopyrene.

[0043] Very clearly, the spectra of the chemical species, made concreteby the intensity and emission wavelength of the emission signalsback-scattered in response to the excitation signals are determinedeither by calculation or, preferably, by experimentation and are storedin databases in the memory of the computer 20.

[0044] As has been shown with anthracene, it is not necessary to comparethe set of the spectrum, which forms a determined surface in space, ofexcitation wavelength, emission wavelength and intensity of the emittedsignal, in order to determine the presence of the chemical species underconsideration but rather simply to carefully select the characteristicexcitation/emission wavelengths and to compare the intensity of theemission signals.

[0045] Nevertheless, when a large number of chemical species could bepresent in the condensed medium and the goal is to detect them, it isadvantageous to proceed to the comparison of a larger portion of thespectrum length of wave by wavelength.

[0046] The identification of a given chemical species can be performedby comparison of the recorded spectrum with the spectrum of saidchemical species stored in the databases by means of any knownidentification program.

[0047] A characteristic of the device according to the invention isbased on the sequential archiving, surface element by surface element,indexed by displacement means 36 and stored in the memory of thecomputer 20, of the measurements of intensity and wavelength of the backscattered signals. Thus, the three variables are stored with twolocalization variables characterized by the relative directions of thebeam of the laser means. In this manner, and taking into account theacquisition rates of the different signals, from each scanning of saidsurface element after scanning of all of the others, it is possible todetect the presence or lack thereof of the chemical species underconsideration and possibly its displacement, by displaying the variablespectrum intensity for the contiguous surface element or element.

[0048] Each chemical species exhibits different durations of fluorescentlife. Thus, by collecting the fluorescence emissions at clearlydetermined times after excitation, it is possible to minimize theinterference between the fluorescence signal and the very short-termemissive phenomena.

[0049] In this manner, in order to evaluate the evolution over time ofthe spectrum of the investigated space, one measures the fluorescenceintensity of each species after a certain delay or during a determinedtime interval, after synchronization of the excitation signal and thedetector.

[0050] Thus, one advantageously determines the characteristic emissionvalues of back scattered electromagnetic emission signals, in responseto an excitation after a given delay and during a given time interval ofat least one chemical species that could be contained in said condensedmedium; one records the intensity values of the back-scatteredelectromagnetic emission signals in response to an excitation of saidcondensed medium after said given time delay and during said period oftime; and one compares said recorded intensity values and saiddetermined intensity values in a manner such as to determine thepresence of said chemical species in said condensed medium.

[0051] In this manner, the time-related resolution of the fluorescencesignal makes it possible to discriminate among the different chemicalspecies as a function of the duration of life of their fluorescenceemission.

[0052] The detection device according to the invention can be installedabove water flows or rivers into which industrial operations dischargetheir effluents in order to continuously monitor the nature of thedischarged effluents and to determine whether the toxic chemical speciesthat they could produce are directly discharged into the environment.

[0053] Other envisaged applications make it possible, for example, tofollow the dynamic evolution of events taking place in a living cell.The chemical constituents that the cell produces, such as proteins, canbe characterized by fluorescence and thus by determined spectra.Consequently, the appearance of a given protein, for example, can bedetected by the device according to the invention.

[0054] Obviously, the optical system positioned between the recordingmeans, notably the detector, and the condensed medium to be explored, iscompletely different when one visualizes a surface portion the dimensionof which are on the order of hundred of meters and when the explorationis directed at a surface portion of a living cell.

1. Method for the detection of chemical species present in a condensedmedium, comprising the following steps: determination of thecharacteristic wavelengths and intensity values of back-scatteredelectromagnetic emission signals due to the fluorescence of chemicalspecies excited in response to a multiplicity of electromagneticexcitations of distinct wavelengths of at least one chemical speciesthat could be contained in said condensed medium; successive excitationof a multiplicity of surface elements of a surface portion of saidcondensed medium with a beam of laser means the tunable wavelength ofwhich is capable of taking on at least one of the values of saiddistinct wavelengths of said multiplicity of electromagneticexcitations; successive recording of the wavelengths and intensityvalues of the electromagnetic emission signals back-scattered by each ofsaid surface elements in response to the electromagnetic excitationsproduced by said beam; comparison at at least one excitation wavelengthand at least one corresponding emission wavelength of the recordedintensity value of said electromagnetic signal back-scattered by each ofsaid surface elements with said determined characteristic intensityvalue of said back scattered electromagnetic signal of said chemicalspecies that could be contained in said surface portion; anddetermination of the presence of said chemical species in each of saidsurface elements when said recorded intensity value of saidelectromagnetic signal back-scattered by said surface element is greaterthan a threshold defined at least by said determined characteristicintensity value of said back-scattered electromagnetic signal of saidchemical species, characterized in that it comprises a prior step ofexcitation of the surface element of the condensed medium with lasermeans and variation of the excitation frequency for a given chemicalspecies in a manner such as to enable detection of the presence of saidgiven chemical species on a portion of the surface of the condensedmedium, the laser beam stemming from the laser means being concentratedon a surface element on a surface portion.
 2. Method for the detectionof chemical species according to claim 1, characterized in that onerecords successively the direction of said beam of the laser means foreach surface element of said surface portion in a manner such as toreference the coordinates of the origin of said back scatteredelectromagnetic emission signals by which one obtains the position ofsaid chemical species in said surface portion.
 3. Method for thedetection of chemical species according to claim 1 or 2, characterizedin that one moreover determines the concentration of said chemicalspecies present in said medium by measuring the amount of energy emittedby said back-scattered electromagnetic emission signals.
 4. Method forthe detection of chemical species according to any one of claims 1 to 3,characterized in that one records in parallel the intensity values ofsaid back-scattered electromagnetic emission signals and in that onerecords their corresponding wavelength.
 5. Method for the detection ofchemical species according to any one of claims 1 to 4, characterized inthat moreover: one determines the characteristic intensity values of theback-scattered electromagnetic emission signals in response to anexcitation after a given interval of time and during a given period oftime of at least one chemical species that could be contained in saidcondensed medium; one records the intensity values of the back-scatteredelectromagnetic emission signals in response to an excitation of saidcondensed medium after said given interval of time and during said givenperiod of time; and one compares said recorded intensity values and saiddetermined intensity values in a manner so as to determine the presenceof said chemical species in said condensed medium.
 6. Device fordetection of chemical species present in a condensed medium for theimplementation of the method according to one of claims 1 to 5,comprising: means (14) for determining the characteristic wavelengthsand intensity values of back scattered electromagnetic emission signals(16) in response to a multiplicity of electromagnetic excitations (12)of distinct wavelengths of at least one chemical species that could becontained in said condensed medium (38); laser means (10) producing abeam (12) for successively exciting a multiplicity of surface elementsof a surface portion of said condensed medium (38) according towavelengths capable of taking on at least the values of said distinctwavelengths of said multiplicity of electromagnetic excitations; meansfor successively recording (14) the wavelengths and the intensity valuesof electromagnetic emission signals back-scattered (16) by each of saidsurface elements in response to the electromagnetic excitations producedby said beam; and comparison and determination means (18), for comparingat at least one excitation wavelength and at at least one correspondingemission wavelength the recorded intensity value of said electromagneticsignal back-scattered (16) by each of said surface elements to saiddetermined characteristic intensity value of said back-scatteredelectromagnetic signal of said chemical species that could be containedin said condensed medium (38) and for determining the presence of saidchemical species in each of said surface elements when said recordedintensity value of said electromagnetic signal back-scattered (16) bysaid surface elements is greater than a threshold defined at least bysaid determined characteristic intensity value of said back-scatteredelectromagnetic signal of said chemical species, characterized in thatthe recording means (14) are connected to a central unit of a computer(20) which presents a memory capable of storing notably simultaneouslythe wavelength of the back-scattered signals (16) and its intensity fora sequential archiving, surface element by surface element, indexed bythe displacement means 36 and stored in the memory of the computer (20)the measurements of intensity and wavelengths of the back-scatteredsignals.
 7. Device for detection of chemical species according to claim6, characterized in that said laser means (10) comprise: a pump laser(22) associated with a frequency doubler; and a parametric oscillator(28) to which said pump laser (22) is coupled in a manner so as to emitradiation the tunable wavelength of which is between 200 and 800 nm. 8.Device for detection of chemical species according to claim 6,characterized in that said laser means (10) comprise a pumping sourceoperating in femtosecond mode.
 9. Device for detection of chemicalspecies according to any one of claims 6 to 8, characterized in thatsaid laser means (10) producing a beam (12) comprise means (36) fororientation of said beam for exciting said multiplicity of said surfaceelements of said surface portion of said condensed medium (38) in amanner to analyze the back-scattered electromagnetic emission signals(16) originating from each of said surface elements and of determiningthe presence of at least one of said chemical species in each of saidsurface elements of said surface portion.
 10. Device for detection ofchemical species according to claim 9, characterized in that itcomprises means for successively recording the direction of said beam ofthe laser means for each surface element of said surface portion in amanner such as to reference the coordinates of the origin of saidback-scattered electromagnetic emission signals such that one obtainsthe position of said chemical species in said surface portion. 11.Device for detection of chemical species according to any one of claims6 to 9, characterized in that it comprises recording means comprising aspectrometer (40) coupled to a matrix of photodetectors (42) in a mannerso as to record in parallel the intensity values of said back-scatteredelectromagnetic emission signals (16) and to record their correspondingwavelengths.